This invention relates to an improved method and apparatus for producing relatively small sulphur pellets in a continuous process. More specifically the invention relates to a method and apparatus for continuously generating sulphur seed particles within the sulphur pelletizing drum.
As exemplified in U.S. Pat. No. 4,272,234 and Canadian Pat. No. 1,148,726, both assigned to the same assignee as the present invention, it is known to produce sulphur pellets by spraying molten sulphur onto a curtain of falling solid sulphur particles to coat and increase the diameter of the particles into product size pellets. Although the method and apparatus disclosed in these patents has been commercially successful, the method and apparatus of the present invention incorporate various design changes which increase the productivity of the system and decrease the cost of the system.
As disclosed in these patents, the heretofore known system includes a rotating drum which rotates a bed of solid sulphur particles. The bed of solid sulphur particles is caused to be elevated by flights associated with the inner surface of the drum so as to cause the solid sulphur particles to descend in a curtain onto the bed. The drum has a pair of deflector plates extending lengthwise of the drum for deflecting the descending solid sulphur particles into two separated, distinct, generally continuous curtains which fall onto the bed at spaced locations. A series of sulphur spray nozzles are spaced longitudinally of the drum beneath one of the deflector plates to spray atomized liquid sulphur particles onto one of the curtains at a pressure in the range of about 200-500 p.s.i. and a temperature above 260.degree. F. The liquid sulphur coats the solid sulphur particles in the curtain of particles to increase the particles in size as they migrate from the feed end of the drum towards the discharge end of the drum.
It is recognized in these patents that it is necessary to maintain the temperature of the bed in the range of about 120.degree. to 180.degree. F. for production of quality pellets resulting from solidification of the liquid sulphur sprayed onto the particles. In order to so maintain the temperature of the bed, a series of longitudinally spaced humidification nozzles are positioned underneath the other deflector plate to spray an atomized liquid onto the second curtain of particles. By so doing, the temperature of the bed is maintained in the range of about 120.degree. to 180.degree. F. Ambient air is drawn through the drum countercurrent to the direction of movement of the bed.
In the heretofore art of sulphur granulation or pelletizing, the temperature of the solid sulphur particles inside the drum was maintained between 120.degree. to 180.degree. F. as a general rule. These temperatures were not high enough to evaporate the water sprayed in the drum. The moist or wet granules being at a temperature lower than the boiling point of water (212.degree. F.) did not evaporate the moisture on the sulphur particles fast enough. Thus, the moist sulphur particles in contact with the small seed nuclei particles had an adverse effect, in that the seed particles stuck to the granules and thereby created an even higher and unpredictable demand for seed particles.
In order to keep the system operating continuously the full sized sulphur pellets which are removed from the system must be replaced with seed nuclei solid sulphur particles. It has heretofore been the practice to supply such seed particles to the drum from an outside source; namely a crusher which crushes oversize particles into seed size particles.
As disclosed in the above identified patents, a method was devised to attempt to generate seed particles within the drum. The seed particles are produced within the drum by intermittently raising the temperature of the bed of solid sulphur particles by at least 40.degree. F. for brief periods. When the temperature of the bed is raised to the range of about 160.degree.-220.degree. F., the liquid sulphur sprayed on the falling curtain of solid sulphur particles does not completely solidify immediately and the sprayed sulphur coating remains slightly soft or plastic on the surface of the particles. As the solid sulphur particles with this plastic coating are tumbled in the bed, the abrading action of the other particles rub or break off small pieces of this soft coating having a diameter in the size range of about 0.1 to 1.0 m.m. After the broken-off pieces solidify, they become the seed nuclei of the system. The temperature of the bed is so raised for approximately 2% of the time to produce the seed particles. It has been found that this method of seed particle production, though feasible, was a cumbersome process from an operational viewpoint. Fluctuating ambient temperatures and changing production rates make use of this method most difficult. Accordingly, it was deemed necessary to develop an improved method of generating seed nuclei particles within the drum on a continuous basis.
Various work has been done by the National Fertilizer Development Center of Tennessee Valley Authority, U.S.A., in the development of urea granulation methods and apparatus. An example of such work is disclosed in U.S. Pat. No. 4,213,924. Reference is made to the discussion in this patent of various prior granulation processes in which seed particles are charged into the bed formed in a rotary drum to produce product size particles. A more recent development by the Tennessee Valley Authority is disclosed in U.S. Pat. No. 4,424,176. The granulation processes referred to above do not contemplate the formulation of seed nuclei particles on a continuous basis within the granulation drum.